5+ “dig” Command Usage Examples in Linux

This tutorial explains Linux “dig” command, options and its usage with examples.

dig – DNS lookup utility

DESCRIPTION

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dig (domain information groper) is a network administration command-line tool for querying Domain Name System (DNS) name servers. Dig is useful for network troubleshooting and for educational purposes. Dig can operate in interactive command line mode or in batch mode by reading requests from an operating system file. When a specific name server is not specified in the command invocation, it will use the operating systems default resolver, usually configured via the resolv.conf file. Without any arguments it queries the DNS root zone.

Dig supports Internationalized Domain Name (IDN) queries. Dig is part of the BIND domain name server software suite. Dig replaces older tools such as nslookup and the host program.

Although dig is normally used with command-line arguments, it also has a batch mode of operation for reading lookup requests from a file. A brief summary of its command-line arguments and options is printed when the -h option is given. Unlike earlier versions, the BIND 9 implementation of dig allows multiple lookups to be issued from the command line.

Simple Usage

A typical invocation of dig looks like:

dig @server name type
where:server is the name or IP address of the name server to query. This can be an IPv4 address in dotted-decimal notation or an IPv6 address in colon-delimited notation. When the supplied server argument is a hostname, dig resolves that name before querying that name server. If no server argument is provided, dig consults /etc/resolv.conf and queries the name servers listed there. The reply from the name server that responds is displayed.name is the name of the resource record that is to be looked up.type indicates what type of query is required – ANY, A, MX, SIG, etc. type can be any valid query type. If no type argument is supplied, dig will perform a lookup for an A record.

The -b option sets the source IP address of the query to address. This must be a valid address on one of the host’s network interfaces or “0.0.0.0” or “::”. An optional port may be specified by appending “#”

The default query class (IN for internet) is overridden by the -c option. class is any valid class, such as HS for Hesiod records or CH for Chaosnet records.

The -f option makes dig operate in batch mode by reading a list of lookup requests to process from the file filename. The file contains a number of queries, one per line. Each entry in the file should be organized in the same way they would be presented as queries to dig using the command-line interface.

The -m option enables memory usage debugging.

If a non-standard port number is to be queried, the -p option is used. port# is the port number that dig will send its queries instead of the standard DNS port number 53. This option would be used to test a name server that has been configured to listen for queries on a non-standard port number.

The -4 option forces dig to only use IPv4 query transport. The -6 option forces dig to only use IPv6 query transport.

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The -t option sets the query type to type. It can be any valid query type which is supported in BIND 9. The default query type is “A”, unless the -x option is supplied to indicate a reverse lookup. A zone transfer can be requested by specifying a type of AXFR. When an incremental zone transfer (IXFR) is required, type is set to ixfr=N. The incremental zone transfer will contain the changes made to the zone since the serial number in the zone’s SOA record was N.

The -q option sets the query name to name. This useful do distinguish the name from other arguments.

Reverse lookups – mapping addresses to names – are simplified by the -x option. addr is an IPv4 address in dotted-decimal notation, or a colon-delimited IPv6 address. When this option is used, there is no need to provide the name, class and type arguments. dig automatically performs a lookup for a name like 11.12.13.10.in-addr.arpa and sets the query type and class to PTR and IN respectively. By default, IPv6 addresses are looked up using nibble format under the IP6.ARPA domain.

To sign the DNS queries sent by dig and their responses using transaction signatures (TSIG), specify a TSIG key file using the -k option. You can also specify the TSIG key itself on the command line using the -y option; hmac is the type of the TSIG, default HMAC-MD5, name is the name of the TSIG key and key is the actual key.

Query Options

dig provides a number of query options which affect the way in which lookups are made and the results displayed. Some of these set or reset flag bits in the query header, some determine which sections of the answer get printed, and others determine the timeout and retry strategies.

Each query option is identified by a keyword preceded by a plus sign (+). Some keywords set or reset an option. These may be preceded by the string no to negate the meaning of that keyword. Other keywords assign values to options like the timeout interval. They have the form +keyword=value. The query options are:

+[no]tcp
Use [do not use] TCP when querying name servers. The default behavior is to use UDP unless an AXFR or IXFR query is requested, in which case a TCP connection is used.+[no]vc
Use [do not use] TCP when querying name servers. This alternate syntax to +[no]tcp is provided for backwards compatibility. The “vc” stands for “virtual circuit”.+[no]ignore
Ignore truncation in UDP responses instead of retrying with TCP. By default, TCP retries are performed.+domain=somename
Set the search list to contain the single domain somename, as if specified in a domain directive in /etc/resolv.conf, and enable search list processing as if the +search option were given.+[no]search
Use [do not use] the search list defined by the searchlist or domain directive in resolv.conf (if any). The search list is not used by default.+[no]showsearch
Perform [do not perform] a search showing intermediate results.+[no]defname
Deprecated, treated as a synonym for +[no]search+[no]aaonly
Sets the “aa” flag in the query.+[no]aaflag
A synonym for +[no]aaonly.+[no]adflag
Set [do not set] the AD (authentic data) bit in the query. This requests the server to return whether all of the answer and authority sections have all been validated as secure according to the security policy of the server. AD=1 indicates that all records have been validated as secure and the answer is not from a OPT-OUT range. AD=0 indicate that some part of the answer was insecure or not validated.+[no]cdflag
Set [do not set] the CD (checking disabled) bit in the query. This requests the server to not perform DNSSEC validation of responses.+[no]cl
Display [do not display] the CLASS when printing the record.+[no]ttlid
Display [do not display] the TTL when printing the record.+[no]recurse
Toggle the setting of the RD (recursion desired) bit in the query. This bit is set by default, which means dig normally sends recursive queries. Recursion is automatically disabled when the +nssearch or +trace query options are used.+[no]nssearch
When this option is set, dig attempts to find the authoritative name servers for the zone containing the name being looked up and display the SOA record that each name server has for the zone.+[no]trace
Toggle tracing of the delegation path from the root name servers for the name being looked up. Tracing is disabled by default. When tracing is enabled, dig makes iterative queries to resolve the name being looked up. It will follow referrals from the root servers, showing the answer from each server that was used to resolve the lookup.+[no]cmd
Toggles the printing of the initial comment in the output identifying the version of dig and the query options that have been applied. This comment is printed by default.+[no]short
Provide a terse answer. The default is to print the answer in a verbose form.+[no]identify
Show [or do not show] the IP address and port number that supplied the answer when the +short option is enabled. If short form answers are requested, the default is not to show the source address and port number of the server that provided the answer.+[no]comments
Toggle the display of comment lines in the output. The default is to print comments.+[no]stats
This query option toggles the printing of statistics: when the query was made, the size of the reply and so on. The default behavior is to print the query statistics.+[no]qr
Print [do not print] the query as it is sent. By default, the query is not printed.+[no]question
Print [do not print] the question section of a query when an answer is returned. The default is to print the question section as a comment.+[no]answer
Display [do not display] the answer section of a reply. The default is to display it.+[no]authority
Display [do not display] the authority section of a reply. The default is to display it.+[no]additional
Display [do not display] the additional section of a reply. The default is to display it.+[no]all
Set or clear all display flags.+time=T
Sets the timeout for a query to T seconds. The default timeout is 5 seconds. An attempt to set T to less than 1 will result in a query timeout of 1 second being applied.+tries=T
Sets the number of times to try UDP queries to server to T instead of the default, 3. If T is less than or equal to zero, the number of tries is silently rounded up to 1.+retry=T
Sets the number of times to retry UDP queries to server to T instead of the default, 2. Unlike +tries, this does not include the initial query.+ndots=D
Set the number of dots that have to appear in name to D for it to be considered absolute. The default value is that defined using the ndots statement in /etc/resolv.conf, or 1 if no ndots statement is present. Names with fewer dots are interpreted as relative names and will be searched for in the domains listed in the search or domain directive in /etc/resolv.conf.+bufsize=B
Set the UDP message buffer size advertised using EDNS0 to B bytes. The maximum and minimum sizes of this buffer are 65535 and 0 respectively. Values outside this range are rounded up or down appropriately. Values other than zero will cause a EDNS query to be sent.+edns=#
Specify the EDNS version to query with. Valid values are 0 to 255. Setting the EDNS version will cause a EDNS query to be sent. +noedns clears the remembered EDNS version.+[no]multiline
Print records like the SOA records in a verbose multi-line format with human-readable comments. The default is to print each record on a single line, to facilitate machine parsing of the dig output.+[no]onesoa
Print only one (starting) SOA record when performing an AXFR. The default is to print both the starting and ending SOA records.+[no]fail
Do not try the next server if you receive a SERVFAIL. The default is to not try the next server which is the reverse of normal stub resolver behavior.+[no]besteffort
Attempt to display the contents of messages which are malformed. The default is to not display malformed answers.+[no]dnssec
Requests DNSSEC records be sent by setting the DNSSEC OK bit (DO) in the OPT record in the additional section of the query.+[no]sigchase
Chase DNSSEC signature chains. Requires dig be compiled with -DDIG_SIGCHASE.+trusted-key=####
Specifies a file containing trusted keys to be used with +sigchase. Each DNSKEY record must be on its own line.
If not specified, dig will look for /etc/trusted-key.key then trusted-key.key in the current directory.

Requires dig be compiled with -DDIG_SIGCHASE.

+[no]topdown
When chasing DNSSEC signature chains perform a top-down validation. Requires dig be compiled with -DDIG_SIGCHASE.+[no]nsid
Include an EDNS name server ID request when sending a query.

EXAMPLES

1. Trace Usage

See how domains are resolved using root servers i.e. turn on tracing of the delegation path from the root name servers for the name being looked up. When tracing is enabled, dig makes iterative queries to resolve the name being looked up.

$ dig +noall +answer google.com
google.com. 5 IN A 173.194.117.41
google.com. 5 IN A 173.194.117.35
google.com. 5 IN A 173.194.117.32
google.com. 5 IN A 173.194.117.40
google.com. 5 IN A 173.194.117.46
google.com. 5 IN A 173.194.117.34
google.com. 5 IN A 173.194.117.39
google.com. 5 IN A 173.194.117.38
google.com. 5 IN A 173.194.117.36
google.com. 5 IN A 173.194.117.33
google.com. 5 IN A 173.194.117.37

$ dig +nocmd +noall +answer a google.com
google.com. 5 IN A 173.194.117.36
google.com. 5 IN A 173.194.117.40
google.com. 5 IN A 173.194.117.34
google.com. 5 IN A 173.194.117.38
google.com. 5 IN A 173.194.117.32
google.com. 5 IN A 173.194.117.37
google.com. 5 IN A 173.194.117.39
google.com. 5 IN A 173.194.117.41
google.com. 5 IN A 173.194.117.35
google.com. 5 IN A 173.194.117.46
google.com. 5 IN A 173.194.117.33

7. Queries may be directed to designated DNS servers for specific records; in this example, MX records: